Conventional coaxial cable comprises a core containing an inner conductor and a dielectric, and a conductive sheath surrounding the core, wherein the sheath serves as an outer conductor. The dielectric is positioned between the inner conductor and outer conductor.
Other transmission lines belong to the class of striplines and its derivatives such as the coplanar line and the microstripline. This class is characterized by a single conductor and one or more ground planes or grounded lines in a planar configuration. The signal travels through the dielectric space defined by these different lines and planes. The dielectric may be homogeneous or consist of different materials, including air.
Various polymeric materials have been used as the primary dielectric material in transmission lines.
One common type of dielectric is polyolefin foam. However, this dielectric is disadvantageous in that dissipation in the dielectric results in signal attenuation. The dielectric losses are a function of the dissipation factor of the polymer, the density of the foam, and the frequency of the signal.
Recently, improvements have been made in polymer foam technology allowing for improved properties of foam which may be used as a dielectric in transmission lines. For example, the MuCell® process of Trexel, Inc. achieves a uniform cell structure having a very small cell size, not more than 100 μm, and frequently not more than 60 μm. The MuCell® process for making microcellular foam is disclosed in U.S. Pat. No. 5,866,053 (“Park”), herein incorporated by reference. In this process, an extrusion system is used to provide the foamed material. The extrusion system involves a supplying plastic pellets which are melted, and also a blowing agent. Specifically, Park uses a supercritical fluid, such as carbon dioxide in the supercritical state, as the blowing agent, to foam a single polymer. In the extruder, the supercritical fluid (carbon dioxide) is dissolved into the molten plastic. The resulting material is then foamed in an expansion stage where a thermodynamic instability is caused by a rapid pressure drop.
In more detail, Park describes an extrusion system for providing a foamed plastic polymer, where a polymer is supplied to an extruder for movement through a rotating screw member. The material is placed in a molten state, and a blowing agent, such as a supercritical fluid, is introduced into the extruder at a selected pressure so that a two-phase mixture of the molten material and the blowing agent is formed. The blowing agent is then diffused into and dissolved in the molten material to form a single-phase solution, which is forwarded from a solution formation area to a nucleation device. Thermodynamic instability is induced through a rapid pressure drop. In a preferred embodiment, a pressure drop greater than 0.9 GPa/s occurs in the nucleation device to nucleate microcells in the solution. A further shaping device, e.g., a die, can be used to produce a foamed material of a desired shape. For further information regarding this process, see <http://www.trexel.com/descript.html>.
By using a polymer alloy which demonstrates high melt strength, superior electric dissipation properties and a glass transition temperature outside the temperature range of −40° C. to 100° C., the low-density foamed dielectric of the present invention is both thermally stable over an extended temperature range and results in extremely low signal attenuation.
U.S. Pat. No. 6,037,545 (“Fox”), herein incorporated by reference, describes a coaxial cable with a foam dielectric. The polymer alloy of the polymer foam is described as a blend of high- and low-density polyethylene. Further, a blowing agent is used in combination with an exothermic nucleating agent, such as azodicarbonamide and an endothermic nucleating agent, such as sodium carbonate/citric acid. The foams described in Fox have densities of between 0.22 g/cm3 and 0.17 g/cm3. However, the foam dielectric of Fox is disadvantageous because foam densities below 0.17 g/cm3 may suffer structural instability and cannot be readily achieved.
The present invention, which uses a combination of a polymer alloy and a supercritical fluid foaming agent, by comparison, can achieve a density as low as about 0.02 g/cm3. In a preferred embodiment of the present invention, the foamed dielectric has a density of from about 0.02 g/cm3 to about 0.20 g/cm3. This translates into superior attenuation properties while preserving favorable structural properties.